I always kept my kittens a good long time, to make sure they had sufficient body fat to weather the adjustment to a new household (a lot of kittens will get a mild illness when adjusting to new food, water, germ pool, etc.) Since Siamese are so thin, they don't have much fat reserve in case of illness.
I never thought about a jacket or sweater because my cats had good clear body color in their younger (showing) years, but a warm jacket would probably lighten the body up -- of course, any Siamese worth her salt would have that jacket OFF and SHREDDED within ten minutes! Here's a funny story though:
This Siamese cat, raised in a cold environment in Moscow in the late 20s, developed a relatively dark coat. An area on his shoulder was shaved, and the cat wore a warm jacket while the fur was growing back. When the shaved hair grew back in, it was white, the same color as the cat's belly, due to the increased temperature under the jacket. This was not due to scarring, as the hair grew in normally colored later.
